Download Last Year`s Exam, Section B

Last Year’s Exam, Section B
Answer any 3 of 5 short questions
5 marks each
 exam is out of 50
i.e. 120/50=2.4 minutes per mark
 hence each question should take ~12 minutes
to answer
do not let yourself get bogged down, but
do not write 2 sentences for 5 marks!
Question B1
Suppose that a solar system exactly like our own
were located about 20 light years away. Using
direct observation (i.e. not by applying theories
of stellar structure), what could astronomers on
Earth learn about this system?
In your answer you should consider properties of the
star, e.g. mass, temperature, chemical composition, and
properties of its planetary system. If you make any extra
assumptions about the system, e.g. its location or
orientation, explain what they are.
B1 Answer
Stellar properties
mass
Planetary properties
 existence
not measurable, system is
not a binary
could probably detect
Jupiter spectroscopically,
if system edge-on
could not detect others
 temperature
from spectral lines or from
colour
 chemical composition
from spectrum
 mass
measure minimum mass
 distance from star
work out assuming mass
for star
 distance
from parallax
 luminosity
from apparent magnitude
plus distance
age
not measurable without
theoretical input
chemical composition
work out Jupiter is a gas
giant, if observe transit
life
might pick up radio
leakage
Question B2
Explain carefully how the following
statements about stars can be justified
from observations:
 Red giant stars have cool surface
temperatures and very large radii
 White dwarf stars are extremely dense
 Globular clusters are very old
B2 answer (i)
Red giant stars have cool surface
temperatures and very large radii
 Red giant stars are red in colour
therefore cool surface temperature
therefore small amount of energy emitted per unit
area
 Red giant stars are very bright
therefore a great deal of energy emitted in total
but not much per unit area
therefore very large area, i.e. very large radius
B2 answer (ii)
White dwarf stars are extremely dense
 White dwarf stars are white in colour
therefore quite hot
therefore a great deal of energy emitted per unit area
 White dwarf stars are faint
therefore little energy emitted in total
therefore small total area, therefore small radius
 White dwarf stars are remnants of Sun-like stars
therefore masses comparable with the Sun (typically about
half a solar mass)
therefore, given small size, must be very dense
B2 answer (iii)
Globular clusters are very old
 The Hertzsprung-Russell diagrams of globular
clusters have a long red giant branch but only
the bottom end of the main sequence
the higher up the main sequence a star is, the
more massive it is and the shorter its main
sequence lifetime
after the main sequence, stars evolve to red giants
therefore with lots of red giants and no upper or
middle main sequence, globular clusters must be
very old
Question B3
What does the visual appearance of the night
sky (as seen through a small telescope) tell you
about the Milky Way?
If you add to the visual information
 the distances of the globular clusters
 the velocity of our Sun relative to the Galactic centre,
and of nearby stars relative to the Sun,
what further statements can you make about the
properties of the Galaxy?
B3 answer
Milky Way appears as a thin band of stars cutting sky in half
 therefore, Milky Way is a disc galaxy
 and we are located near midplane of disc
Blue stars and dust clouds are seen
 therefore, star formation ongoing in disc
Band is brightest
around Sagittarius
 therefore, this is
direction of
centre
B3 answer, continued
Add globular cluster
distances
 confirm centre in
direction of Sagittarius
 determine distance of
centre
Add velocity info
 determine mass of
Galaxy inside Sun’s
orbit
 find that orbits of disc
stars are highly
correlated
disc is a rotating system
disc stars in nearcircular orbits
Question B4
Explain how the cosmic microwave
background was generated, and briefly
discuss what its properties tell us about
the Universe and its history.
B4 answer
Cosmic microwave background is thermal
radiation (it has a blackbody spectrum)
 this spectrum was produced when universe was hot,
dense and ionised (and therefore opaque) – radiation
and matter in equilibrium
 microwave background as we see it dates from era
when protons and electrons combined to form neutral
hydrogen: universe became transparent (~300000
years after Big Bang)
 temperature then was ~3000 K: present temperature
of ~3 K comes from expansion of universe by factor
1000 since that time
B4 answer, continued
What does CMB tell us?
 thermal spectrum implies whole universe once hot,
dense, ionised, at specific time in past
expected in Big Bang theory
contrary to basic assumptions of Steady State theory
 extreme uniformity suggests that whole visible
universe was once in thermal equilibrium
very difficult to understand in standard Big Bang
expected from inflation
 detailed properties can tell us values of cosmological
parameters
WMAP data give Hubble constant, geometry, density, value
of cosmological constant,…
Question B5
Explain the concept of habitable zone
when applied to extrasolar planetary
systems.
What factors enter into estimates of the
number of technological civilisations in the
Galaxy?
 Briefly discuss whether it is possible to make
accurate estimates of the values of these
factors.
B5 answer
Habitable zone
 range of distances from star at which Earthlike planet could support liquid water (i.e.
would have surface temperature 273 – 373 K)
 strictly should allow for stellar evolution on
main sequence (continuously habitable zone)
B5 answer, continued
Factors entering estimate
 number of suitable stars (or rate of formation of
suitable stars)
 fraction of those stars with planets
 fraction of those planets which are Earth-like
 fraction of Earth-like planets evolving life
 fraction of life-bearing planets developing intelligence
 fraction of intelligent species developing technology
 average lifetime of technological civilisation
 = estimate now;  = could in future estimate;
 = hard/impossible to estimate
Last Year’s Exam, Section C
Answer any 1 of 3 long questions
15 marks each, ~36 minutes’ work
Question C3 is on the seminars:
 Write short essays on any three of the
following
binary stars
black holes
the search for dark matter
the effects of asteroid and comet impacts on Earth
Question C1
Briefly explain how nuclear fusion processes
generate energy, and why we believe that main
sequence stars are powered by hydrogen fusion.
 Energy generation:
for elements up to iron, heavier nuclei are more tightly bound
(hence less massive) than lighter nuclei
hence, if light nuclei are fused to make heavy nucleus, extra
mass is converted to energy via E=mc2 (release of binding
energy)
 Powering of main sequence stars:
hydrogen fusion most efficient (0.7% of mass converted)
hydrogen fusion easiest (fastest moving, least charge)
hydrogen by far most abundant element
hydrogen fusion will start at lowest temperature and give
longest stellar lifetimes
C1 continued
The Orion Nebula is a well-known region
of star formation containing a number of O
and B class stars. The brightest star in the
Orion Nebula is θ1 Orionis C, which is
nearly one million times as bright as the
Sun and is the brightest main-sequence
star known in the Galaxy.
C1 (a)
Explain why very bright main-sequence stars like θ1
Orionis C are always found in or near star formation
regions, whereas less bright main-sequence stars like
the Sun can be found anywhere.
 Brighter main-sequence stars are more massive.
 Luminosity increases much faster than mass: a star 10 times as
massive is 10000 times as luminous.
 Therefore massive stars last for much shorter time on main
sequence (poorer ratio of power used to fuel available!)
 Therefore the very brightest, and shortest-lived, stars have no
time to move away from the region in which they were formed
(and no time for the star formation region to run out of gas!)
C1(b)
Describe how θ1 Orionis C will evolve in
the future.
 What will happen to it in the end?
 What effect will this have on any stars which
may subsequently form in the Orion Nebula?
C1(b) answer
Currently on main sequence (i.e. fusing hydrogen to
helium in core)
when hydrogen runs out in core, star shrinks under gravity
until hydrogen just outside core is hot enough to fuse
 star expands and cools, becoming red (super)giant
helium core gets more massive and hotter until it
eventually fuses to carbon
 star gets smaller and bluer again
 subsequently helium fusion moves out from core, star becomes
red giant again (fusing helium around carbon core)
this is a massive star, so fusion continues beyond helium
 star fuses successively heavier elements until it develops iron core
 each successive stage takes less time than the one before
C1(b) continued
What will happen to it in the end?
 iron fusion does not generate energy
when iron core gets too big, it will collapse, and
cannot be saved by fusion
iron core collapses to neutron star (or, for star as
massive as θ1 Orionis C, perhaps black hole)
infalling outer regions bounce off rigid neutron star
surface
 star explodes as supernova
C1(b) continued
What effect will this have on any stars which
may subsequently form in the Orion Nebula?
 outer regions of star contain heavy elements made
during star’s life and during supernova explosion
 explosion disperses these into surrounding interstellar
gas
 therefore, stars forming from this gas will have greater
heavy element content than stars which formed
earlier
C1(c)
Suppose that you could observe the Orion
Nebula region after the death of θ1 Orionis
C. Describe the remnants of θ1 Orionis C
that you might see.
 supernova remnant
expanding cloud of gas, cf. Crab Nebula
 compact object
neutron star
 visible as pulsar (rapid regular pulses of radio, visible
and X-ray emission) if viewed from correct angle
black hole
 possibly visible via accretion disc
Question C2
Explain the “Hubble tuning fork” classification of galaxies.
 Main division: elliptical galaxies, spiral galaxies and
irregular galaxies
elliptical galaxies E0 – E6 based on shape (higher number =
more elongated)
spirals either normal (S)
or barred (SB)
 subclasses a–c based on
 relative brightness of
bulge (brightest in a)
 winding of spiral arms
(loosest in c)
 S0/SB0: disc galaxies
without spiral arms
irregular galaxies have
amorphous or disrupted
structure
C2 continued
The Andromeda galaxy is moving towards the Milky
Way and may collide with it in a few billion years.
Discuss what would happen in such a collision, and
what the results would be.
 What would happen:
disruption of orbits of stars and gas, and therefore of disc
formation of tidal tails
large increase in star formation
probable eventual merger
 Result:
large merged galaxy with no disc and
little remaining gas: elliptical galaxy
C2 continued
You observe two large clusters of galaxies,
one nearby (e.g. Coma) and one very
distant. How does the distant cluster differ
from the nearby one?





spectrum has a large redshift
more interacting galaxies
fewer spiral galaxies
more small blue galaxies
more likely to include an active galaxy
C2 continued
Explain the significance of these differences for
theories of galaxy evolution and for cosmology.
 spectrum has a large redshift
distant galaxies are receding from us: universe is expanding
 fact that clusters look different at all
the universe is evolving: it has not looked the same at all
times in the past
contradicts Steady State theory
 larger numbers of small and interacting galaxies
supports idea that mergers and interactions play important
role in galaxy evolution (especially in rich clusters)